Lead coating process



Patented Oct. 11, 1949 2,484,540 LEAD COATING PROCESS South Euclid, Ohio, as-

Irving P. Whitehouse,

Signor to Republic Steel Corporation,

Cleveland,

Ohio, a corporation of New Jersey No Drawing. Application October 3, 1945, Serial No. 620,176

4 Claims. (Cl. 20436) The present invention relates to the art of coating metal articles with lead and is particularly concerned with a new method of making lead coated articles which are substantially free from porosity or pin-holes in the coating and are highly resistant to corrosion of the underlying metal.

Electrolytic deposits of lead on ferrous metal articles have been used heretofore to protect such articles against corrosion but the protection realized has not been found to be entirely satisfactory. In general, it may be said that the ferrous metal was corroded in small isolated areas and that the service life was much shorter than was desired when relatively heavy coatings were employed.

It is well known that the cause of such short life and such localized corrosion is traceable to discontinuities in the lead coating generally in the form of pin-holes or pores and that when such pin-holes are closed the underlying ferrous metal is protected against corrosion over substantially its entire coated surface for much longer periods of time.

The present invention provides a new method of making electrolytic lead coatings which are substantially free from pin-holes and its use provides lead coatings which will prevent corrosion of the underlying metal for at least twice as long in accelerated corrosion tests as the prior electrolytic lead coatings of the same thickness. Stated briefly, the present invention comprises the important step of burnishing an electrolytic lead coating in a liquid having lubricating qualities.

Lead may be deposited electrolytically on ferrous metal by several well known procedures. One such procedure is as follows: Ferrous metal is degreased by treating it with a suitable organic solvent, for example, trichlorethylene or carbon tetrachloride, after which the metal is electrolytically cleaned in an alkaline cleaner. Here the metal may be made the anode, cathode, or alternately anode and cathode in the electrolyte. Current density of from 20 amps. to 40 amps, per sq. it. may be used while the electrolyte temperature is between 180 F. and 200 F. After completion of this electrolytic cleaning the metal is ordinarily rinsed or scrubbed in water to remove oil smut and the like from its surfaces which sometimes are present when the metal was not adequately degreased in the organic solvent. Then the metal is usually given a short acid pickle, for example, in an aqueous bath containing from 8% to 10% by weight of sulfuric acid and at a temperature 2 of 165 F. to 180 F. Ordinarily only a few seconds are required for this pickling operation but longer times may be necessary where heavy scale is present on the metal. Then the metal is again rinsed and scrubbed to remove foreign material from the surface thereof and is immediately immersed in the plating tank so as to prevent oxidation so far as possible.

One well known plating solution is an aqueous electrolyte containing about 64 oz. of lead sulfamate per gallon and about 1.35 oz. per gallon of an addition agent. In still plating with such an electrolyte the bath temperature may range from F. to F. with a current density of from 20 amps. to 40 amps, per sq. ft., the pH value of the bath being maintained between 1.7 and 2.0.

Frequently another step is included between the foregoing pickling and lead plating steps. In this additional step the clean and pickled metal is given a flash coating of electrolytic copper before it is electroplated with lead. This copper coating may approximate 10-millionths of an inch in thickness and may be made from an aqueous electrolyte containing, per gallon, about 2 oz. of cuprous cyanide, about 4 oz. of sodium cyanide, and about 2 oz. of potassium hydroxide, the temperature of the electrolyte being maintained at between F. and F. and current densities of from 20 amps. per sq. ft. to 30 amps. per sq. ft. being used. After the article is so plated with copper it is rinsed with water, and is then electrolytically plated with lead as previously described.

The article coated with electrolytic lead, and whether embodying the copper flash coating or not, is rinsed with Water and immersed immediately in an alkaline chromate solution for from 5 to 10 seconds. A suitable chromate solution consists of water containing about 10% by weight of sodium chromate and about 1% by weight of sodium hydroxide. Then the article is again rinsed in water, which is preferably hot to expedite drying, or it may be immersed in acetone or alcohol for two or three seconds and then exposed to air where it dries rapidly.

Lead fluoborate electrolytes are also commonly used instead of the lead sulfamate electrolyte above described. When such lead electrolytes are used they may be aqueous solutions containing, per gallon, about 40 oz. of lead fluoborate, about 3 oz. boric acid, about 4 or 5 oz. of free fluoboric acid and about .04 oz. of an additional agent such as bone glue. The current density ranges between about 20' amps. and about 40 amps. per sq,

tain lubricant or .not.

ft. and the bath temperature is about 100 F. and its pH value is from 1 to 1.5 for still plating.

However the electrolytic coating of lead is formed, whether by one or the other of the foregoing methods or some other method, the lead coated article according to the present invention is then burnished in a lubricant. Preferably, rolls which are covered with cloth or other soft fibrous material are rotated rapidly while being pressed against the electrolytic lead coating and while'a mineral or vegetable oil is in contact with the lead.

In general it may be said-that any natural or synthetic liquid possessing some lubricating qual- I lecithin.

I am not certain as to exactly what takes place during this burnishing .operation but I believe that the burnishing rolls wipe the lubricant into ;the poresof thelead coating and additionally wipe the surfaces of the lead in such a way as to close the pin-holes whether they actually con- I-Iowever, lead coatings treated in this "manner have been found to be substantially-free frompin-holes and to be quite resistant to corrosion and to withstand accelerated corrosion tests on the order of twice as long as electrolytical coatings not so treated.

, Apparently both the burnishing action and the presence of lubricant are important in obtaining the desired results for neither merely dipping the electrolytically coated article in the same lubricant nor burnishing in the absence of lubricant showed any appreciable improvement in corrosion resistance. However, when the burnishing was carried out in the presence of lubricant the rateand extent of corrosion was greatly reduced as evidenced by the extended service life.

Having thus described the invention so that others skilled in the art may be able to understand and practice the same, I state that what I desire to secure by Letters Patent is defined in what is claimed.

said coating with soft, fibrous material while the .coating is in a mineral oil.

2. The process of lead coating a metal article which comprises the steps of electrodepositing a coating of lead on such an article and closing pin holes in the coating and increasing the resistance of the article to corrosion by frictionally wiping said coating with soft, fibrous material while the coating is in avegetable-oil. v

3'. The processof makinga lead coated metal article which comprises the steps of electrodep'ositing a coating of lead on the clean surface of such an article and closing pin holes in the coating and increasing the resistance of the article,to.corrosion.byfrictionally wiping said coating with soft fibrous material while the coating is in a liquid'lubri'cant.

4. The process of making a lead coated metal f'articldwhich comprises the steps of electrodeposi ting a coating of lead on the clean surfaces of-such an article and closing pin-holes in the coating and increasing :the resistance of the ar- "ticle .to corrosionby frictionally wiping the surface of the coating with cloth in'a liquid lubricant. 1 IRVING WHITEI-IOUSE.

, REFERENCES CITED The following references are of record in the while the coating is "file of this patent:

UNITED STATES PATENTS Number Name Date 685,211 Kennedy -4; Oct. 22, 1901 1,580,198" Harrison Apr. 13, 1926 1,869,041 Bengston July 26, 1932 2,126,578 Roemer et a1. Aug. 9, 1938 2,141,382 Ferm Dec. 27, 1938 2,177,316 Giles Oct. 24, 1939 2,318,592 Cupery "May 11,1943 FOREIGN PATENTS Number Country Date 2,460 Great Britain 1 1881 130,302 Great Britain 1919 484,103 Great Britain 1938 639,701

QfII-IER REFERENCES ProtectiveGoatings for'Metals, by R. M. Burns and A. E. iSchuh', .published'by Reinhold Publishing Co.,1New,- York, 1939, pp. 34, 207, 208.

Iron Age, Apr. 30, 1942, pp. 29-44. v

France -Mar. 17, 1908 

